1. Field of the Invention
The invention involves communication systems in which a number of users share a limited-capacity communications medium, and more particularly, the invention involves techniques for optimizing the number of users who can use such a medium concurrently.
2. Description of Related Art
Many industries require effective, low-cost communications capability within a local area among many physically separated, mobile workers. Such capability is important for large construction projects, convention and conference centers, major entertainment and sporting events, fire and other emergencies, service business and sales offices, high-end retail stores, airports and process manufacturers, for example.
In many of these kinds of sites, the nature of the work is one of unpredictable and constantly changing patterns of activity. Effective on-site and off-site voice communications are particularly important to the supervisor's ability to coordinate this work and to quickly resolve frequent, unexpected problems which can have significant financial impact. In crisis management and certain tasks on construction sites, such as crane operation, high communication reliability is critical to safety.
In the particular case of construction sites, a construction superintendent or foreman makes numerous decisions each day. The limitations of job-site communications, however, result in many of these decisions being made without complete information, or being put off until more information or instructions can be obtained. Either outcome can negatively impact the construction cost and schedule. The limitations of existing on-site communication systems also often cause delays as workers track down other personnel in order to obtain information, or have to re-do work because of improper instructions, all resulting in substantial productivity loss directly related to the poor job-site communications of existing systems.
Workers on sites such as these are not currently offered a flexible, integrated, cost-effective wireless voice communication solution. Some combination of general purpose communication products, such as cellular, wireline and cordless phones, pagers and broadcast radios, are currently being used with limited effectiveness. Pagers have limited effectiveness because they allow communication of only a small amount of information at a time, thereafter requiring the worker to leave his or her position and go elsewhere (such as to a telephone) to communicate further. Wireline and cordless phones with local switching systems allow a number of private conversations to occur concurrently, but severely limit the range with which a worker can move and still continue the conversation. Cellular phones, which time--or frequency--domain multiplex a number of phones in a star-type configuration relative to a base station, allow multiple private communications with a wide range of movement, but are expensive to operate and suffer from peak use and periodic reliability limitations. No solution is available today which economically allows multiple concurrent private communications to occur among workers who need freedom of movement around a large work site.
Broadcast radio systems are often used for local on-site communications. But in many installations all radios reach all the other radios on the same frequency. Because no conversations are private, many conversations cannot use the system and must wait until two people meet face-to-face. Additionally, because a user must constantly listen for a call, the user needs to suffer the distraction of keeping the radio on at all times. Many broadcast radio installations also operate in a "single-speaker model", in which one party must stop speaking before another can begin. Substantial productivity can be lost, therefore, as users wait for air time. Multi-channel radios and directed calling features reduce this problem in existing systems somewhat, but add the complexity of requiring two users to coordinate channel changes, during which time they are out of touch with the common channel.
For voice communications, systems exist which allow a larger number of users to share the communications medium, through the use of data compression techniques. A variety of data compression techniques are available, but in general, the greater the compression, the more the original voice quality suffers. In general, systems are designed for a single compression level, as appropriate for the particular market. The compression level remains constant irrespective of available bandwidth, and irrespective of caller's priority. Thus a system designed to support many users might be designed to use very heavy compression. Voice transmission quality can suffer so drastically in such systems as to render the speaker unrecognizable.
Accordingly, there is a distinct need for an economical, local area communications system which can support a large number of private conversations concurrently, while maximizing the audio quality of such communications.